This invention relates to a piston ring adapted for a piston disposed in an internal combustion engine, and more particularly, to a compression ring made of steel material adapted for such a piston.
Usually, in an internal combustion engine, there is disposed a piston cylinder assembly having a piston provided with a plurality of annular piston grooves formed on the outer periphery thereof, and piston rings including a compression ring and an oil ring are fitted into these piston grooves so that sliding surfaces of the piston rings contact slidingly the inner wall of the cylinder when the piston reciprocates therein.
Recently, in accordance with social needs, there have been developed high performance internal combustion engines capable of high revolutions and having high power or high compression ratio, including superchargers. Hence, it is required to provide piston rings to be utilized for such internal combustion engines having improved structures and grades. Further, in one aspect, it is recommended to use unleaded fuel from a viewpoint of environmental pollution, but lead including fuel has yet been used relatively widely in various countries. The interior of a piston cylinder assembly of the internal combustion engine which utilizes the lead containing fuel is strongly dominated by corrosive atmosphere or environment such as HCl or H.sub.2 SO.sub.4. In order to suppress as much as possible abrasion of the piston ring caused by the corrosive environment, piston rings having sliding surfaces subjected to a nitriding treatment have been developed to provide anti-abrasion and anti-corrosion properties to the sliding surfaces of the piston rings. However, when the piston ring subjected to the nitriding treatment is used for the piston cylinder assembly of a high performance internal combustion engine power use, it is required for the piston ring to have anti-breaking property.
In a conventional compression ring as a piston ring for an internal combustion engine, the piston ring is composed of a steel base member having a surface on which is formed a nitriding layer by the nitriding treatment. In accordance with such nitriding treatment, a compound layer, called white layer hereafter, in a porous form, having very hard and brittle property is formed on a surface portion of the nitriding layer, and only the porous white layer formed on the sliding surface of the piston ring is removed by an after-treatment. In such a manner, a piston ring is manufactured as a product. Accordingly, the hard and brittle porous white layer remains on corner portions or upper and lower surface portions including corner portions of the base member of the thus produced product.
There is known a material of the base member having a composition of, for example, in weight %, C: 0.85 to 0.95, Si: 0.35 to 0.50, Mn: 0.25 to 0.40, Cr: 17.00 to 18.00, Mo: 1.00 to 1.25, V: 0.08 to 0.15, and a balance of Fe and inevitable impurity, or a composition of, for example, in weight %, C: 0.87 to 0.93, Si: 0.20 to 0.40, Mn: 0.20 to 0.40, Cr: 21.00 to 22.00, Mo: 0.20 to 0.40, Ni: 0.90 to 1.10, and a balance of Fe and inevitable impurity.
When the compression ring of the type described above is used by fitting the same into a piston groove formed to the piston of the internal combustion engine, the compression ring will repeatedly collide with the piston groove by contracting or expanding motions in the radial direction of the piston ring and the vertical motions in the axial direction thereof. During such motions, if the porous, hard and brittle white layers remain at the corner portions or near the corner portions of the base member of the compression ring, cracks will be formed at those portions. The cracks then progress and finally may result in the breaking of the compression ring.